1. Field of the Invention
The invention relates to a signal processing method and module, more particularly to a signal processing method and module that can preserve phase characteristics of a signal to be processed thereby.
2. Description of the Related Art
A conventional digital signal processing method for processing audio and video signals usually adopts Hilbert Transform or Fast Fourier Transform (FFT) to transform data into magnitude and phase characteristics in the frequency domain. However, in cases where the phase characteristics of signals are not allowed to be transmitted, or when only specified phase characteristics of signals are transmitted, the original signal characteristics are not preserved during processing.
In the field of audio signal processing for cochlear implants, due to limitations to activation patterns of nerves and ganglion cells, current techniques only allow transmission of magnitude characteristics in the frequency domain and extraction of important voice features, such as transmission of fundamental frequencies and associated overtones, or finding of larger frequency domain signals (“Better speech recognition with cochlear implants”, Wilson et al., Nature 352, p. 236-238, 1991; “Chimaeric sounds reveal dichotomies in auditory perception”, Smith et al., Nature 416(6874), p. 87-90, 2002). In other words, phase characteristics, which are difficult to process, as well as noise and low-volume overtones, which are deemed to contain unimportant information to humans, are therefore discarded. Nevertheless, in order for the human central nervous system to recognize language contexts, musical melodies, sound sources, sound directions, etc., phase characteristics at various frequencies are factors that are too important to be neglected and are therefore required to be at best preserved during transmission.
The following are some methods relevant to the art of signal processing:
1) only those input frequency domain data that cross certain phases are transmitted:
In U.S. Pat. No. 6,480,820, there is disclosed a method of processing auditory data in which, after transformation of an audio signal into frequency domain data, those frequency domain data with no axis crossing are discarded. However, in practice, frequency domain data belonging to low frequencies (under 2000 Hz) and having no axis crossings actually form a large part of the audio signal. Hence, by discarding the frequency domain data having no axis crossings, most of the audio signal is actually lost. Moreover, in practical applications (especially in high frequency applications), it is possible that there may be no smooth axis crossings under certain analytical conditions. If interpolation is relied upon to generate frequency domain data with axis crossings for two frequency domain data with a large phase difference therebetween, distortion of the audio signal will result.
2) Traveling waves of basilar membrane response are simulated:
In U.S. Patent Publication No. 20030171786, acoustics signals are first transformed using FFT so as to obtain a plurality of data, each of which includes frequency magnitude and phase characteristics. Thereafter, by simulating the progress and delay phenomena of traveling waves, output signals are able to stimulate the cochlea at more accurate times. However, since imaginary data components are discarded during signal processing, phase characteristics and a part of magnitude characteristics are thus discarded. As a result, the original signal features are altered, making it impossible to reconstruct the original acoustic signal.
3) Simultaneous fast stimulation of electrodes:
In U.S. Pat. No. 6,504,525, there is disclosed a method of activating electrodes in a multi-channel electrode array of a cochlear implant using channel specific sampling sequences. However, in the disclosed method, the phase characteristics are also discarded during processing, which alters in part the features of the original audio signal.
Furthermore, in U.S. Pat. No. 6,647,297, in a typical method of signal processing in a retinal implant device, it is not allowed to transmit phase characteristics of spatial frequencies of an image signal. Since the phase characteristics are assumed to be equal, only magnitude characteristics of brightness or color stimulus are maintained.
Phase characteristics are critical to preserve the clarity of original audio and video signals (“The importance of phase in signals”, Oppenheim et al., IEEE, Proceedings, vol. 69, p. 529-541, 1981). Therefore, if phase characteristics at various frequencies can be preserved and transmitted, the extent of recognizability of reconstructed audio and video signals will be dramatically improved.
Moreover, in orthogonal frequency division multiplexing (OFDM) modulation systems or other systems that require multi-phase data transmission, there is a need for a signal processing method in which original multi-phase signals are integrated into predefined phase angles so as to permit transmission of larger volumes of data.